1. Field of the Invention
The present invention relates to an image forming apparatus in which a suction conveying portion to suction and convey sheet is arranged between a transfer portion to transfer toner images and a fixing unit to fix toner images to the sheet.
2. Description of the Related Art
As is well-known, in an image forming apparatus utilizing an electrophotographic system, a recorded image is obtained by transferring a toner image being a visible image which is borne on a photosensitive member or an intermediate transfer member to transfer material (sheet) such as plain paper. Then, the transfer material to which the toner image is transferred from the photosensitive member or the intermediate transfer member at the transfer portion is discharged after the toner image is fixed as being conveyed to a fixing unit. If the transfer material is conveyed with a pair of normal rollers between the transfer portion and the fixing unit, the non-fixed toner image on the transfer material is damaged. Accordingly, as a structure for transfer material conveying, it is generally structured that a suction conveying portion conveys as suctioning transfer material to an endless belt having a number of suction air holes as suctioning with a fan.
Between the transfer portion and the fixing unit, for example, there may be a case that transfer material is not nipped by neither a transfer nip portion nor a fixing nip portion after being discharged from the transfer portion until conveyed into the fixing unit. In such a case, the transfer sheet must be conveyed only with suction conveying force of the suction conveying portion.
Transfer material having a curl is difficult to be suctioned to the endless belt at the suction conveying portion. Accordingly, there may be a possibility that conveying force cannot be applied to the transfer material and that the transfer material cannot be conveyed to the fixing unit. Further, even in a case that the transfer material is nipped by one of the transfer nip portion and the fixing nip portion, there may be a possibility that a curl at the transfer material causes difficulty of suctioning to the endless belt as well when suctioning force of the suction conveying portion is relatively weak.
Japanese Patent Laid-Open No. 2003-327342 discusses a structure in which a drive roller is protruded upward from a conveying face of an endless belt at the downstream side in the transfer material conveyance direction from the suction conveying portion and conveys a curled transfer material to the fixing unit with the drive roller.
However, the above image forming apparatus in the related art assumes a case that the transfer material front end is curled downward. Actually, a curled direction of a transfer material front end varies according to a state of transfer material set by a user and fixing conditions, conveying path shapes after fixing and an amount of transferred toner for a second face of duplex printing. Accordingly, the curled direction is not uniformly defined.
Accordingly, with the traditional structure in which the drive roller is protruded upward from the conveying face of the endless belt when the transfer material front end is curled upward, the transfer material front end is more unlikely to enter to the fixing unit. Therefore, a conveying error may be caused on the contrary. Even when a conveying error is not caused here, a non-fixed toner image on the transfer material grazes with a guide of the fixing unit to cause an imaging error.